A decade of both epidemiologic and biologic research has firmly established Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) as the infectious cause of the tumor that shares its name. Infection with this gammaherpesvirus can also lead to two other human diseases, multicentric Castlemen's disease, a lymphoproliferative disorder, and primary effusion lymphoma, a B cell tumor most often afflicting persons with acquired immune deficiency syndrome (AIDS). Although initial KSHV exposure is generally asymptomatic and likely results in the latent infection of a limited number of cells in the host, immunosuppression can lead to unchecked viral spread, expansion of latently infected cells and eventual tumor formation. This pathogenic progression as well as person-to-person spread depends on lytic reactivation and replication of virus followed by recruitment, infection and hyperplastic expansion of a critical mass of new target cells. The most remarkable structures to appear following the initiation of lytic replication are the icosahedral capsids that fill the nucleus and, when fully mature, harbor the linear viral genome. Our NIH-sponsored work on KSHV during the initialfunding period was the first to explore simultaneously the protein composition, stoichiometry andthree-dimensional structure of capsids from a gammaherpesvirus. We propose in this application to extend these studies toward a comprehensive understanding of the composition and structure of the virions, examining both KSHV and its useful primate homolog, Rhesus monkey rhadinovirus (RRV). Since virions must contain the proteins essential for host range determination and initiation of infection, it is critical to comprehensively identify their protein composition. Moreover, knowledge of this composition is a prerequisite for functional investigations. Wewill integrate molecular and imaging techniques to identify the virion-associated proteins and the genes encoding them while characterizing the finer details of the viral architecture. The results of our proposed studies on viral structure, molecular composition and assembly may help identify new targets for therapeutic intervention in the future.